Method From the Central Denmark Region, the investigators will obtain social security numbers of all gastric bypass operated persons in the Central Denmark Region (approximately 3500 patients). From the State Serum Institute will be obtained information on their address from the social security number and they will be invited by letter to participate in the survey. The letter will contain a numbered questionnaire hence the answer is anonymous. The questionnaire will include questions relating to weight loss, surgical complications, possible remission of diabetes, substitution treatment, nutritional complications, hypoglycaemia, quality of life (SF-12), etc.